1. Technical Field
The present invention relates to the processing of planar substrates and more particularly relates to holders for substantially planar workpieces, providing non-contact support for typically thin, flexible workpieces without incurring substantial distortion of the workpiece or risk of fracture.
2. Description of Related Art
Numerous areas of technology require that a workpiece be held in position while processes are performed thereon. In particular, the processing of semiconductor wafers into integrated circuits requires the wafer to be held by a suitable holder without impeding the processing steps directed to the exposed flat surface of the wafer. Following production of the integrated circuits, wafer processing is generally not finished. In particular, many areas of technology (cell phones, smart cards and the like) require that the integrated circuit providing the internal intelligence for the device be very thin. It is typically inconvenient to fabricate the integrated circuits directly on a thin wafer as distortion, non-uniform processing, perhaps even rupture, may occur during the numerous processing steps typically required for integrated circuit fabrication. Therefore, it is common practice for a wafer containing numerous fully fabricated integrated circuits thereon (on the xe2x80x9cfacexe2x80x9d side of the wafer) to be thinned by means of etching from the reverse side (xe2x80x9cback sidexe2x80x9d) of the wafer. Plasma etching is a typical method for wafer thinning. For economy of language we will refer to the xe2x80x9cback sidexe2x80x9d of the wafer and intend thereby the face undergoing (or destined to undergo) thinning by etching. The xe2x80x9cfacexe2x80x9d of the wafer denotes the face of the wafer not undergoing etching, typically having the integrated circuits thereon and lying in close proximity to the wafer holder.
To be definite in our description of the invention, we will describe the common instance of the processing of semiconductor wafers as may typically occur in the production of integrated circuits or in wafer post-processing for thinning, etc. However, the processing of any flexible planar workpiece in a non-contact holder is also a potential area of application for the present invention. Substrates for flat panel displays, printed wire boards, various films (possibly having layered structures), construction materials such as plywood, paneling and the like may require processing while held in a suitable holder. Other rectangular, circular, star-shaped or irregularly shaped planar workpieces may require processing by means of a non-contact holder. Such workpieces may be relatively large (perhaps 10 meters across). In addition, such workpieces may be flexible, readily subject to distortion or damage in the holder. The present invention provides non-contact support for such objects while avoiding unacceptable distortion. To be definite in our description, we will discuss the commercially important processing of the back side of semiconductor wafers. However, generalizations to other workpieces requiring non-contact support without substantial distortion will be obvious generalizations of the present invention.
Since semiconductor wafers are expected to be an important area of application for the present invention, we will describe the primary features of the present invention in terms of semiconductor processing, not intending to limit the invention to this particular choice or particular example. Semiconductor wafers aptly illustrate the features of the present invention and permit obvious modification for use in processing other flexible workpieces.
Several challenges must be met for successful wafer thinning. The wafer is (or soon becomes) very thin, rendering it susceptible to distortion. Distortion would generally lead to non-uniform etching, non-uniform heating of the wafer (as during plasma etching, for example) and potential damage to the integrated circuits lying on the face of the wafer opposite from the back side being etched. Thus, an important challenge to be met by a wafer holder is the ability to hold thin, easily distorted wafers in a flat position during etch. Of course, the wafer holder must not contact the exposed integrated circuits lying on the face of the wafer nearer the wafer holder. Thus, non-contact support for a flexible wafer must be provided.
Another challenge to be met by the wafer holder relates to preventing debris from the plasma etch (for example) from contaminating the integrated circuits on the face of the wafer. More stringently, the wafer holder should also prevent debris from impacting the thin edge of the wafer. Non-contact support invariably implies a gap between the wafer and the holder. Etching debris clearly need to be kept out of this gap. Preferably, the etching debris should also be kept from the wafer edge. While not as serious a problem as debris contacting the opposite face of the wafer, edge contamination may lead to rejection of the particular integrated circuits lying on the contaminated regions of the edge. Thus, integrated circuit yield would be reduced.
The floating of a wafer above a layer of compressed gas is described in the work of Pirker (U.S. Pat. No. 5,896,877). The wafer is held in position by gravity while the air cushion prevents contact with the wafer holder.
Siniaguine and Steinberg (PCT International Publication No. WO/97/45862, hereinafter xe2x80x9c""45862xe2x80x9d) describe a non-contact holder for wafer-like objects in which a vortex of rotating air provides both the vacuum support for the wafer while providing the air cushion insuring non-contact with the wafer holder. The present invention makes use of a vortex-created partial vacuum to provide both wafer support and an air cushion insuring non-contact, including several improvements to the technology of ""45826 as described in detail below.
The present invention relates to a non-contact holder for flexible workpieces, particularly suited for holding thin workpieces such as semiconductor wafers without substantial distortion. One feature of the present invention is a cylindrical chuck having a gas inlet orifice therein positioned at an oblique location with respect to the central axis. The introduction of pressurized gas through this orifice leads to the creation of a vortex within the chuck. The vortex creates vacuum attraction to hold a wafer in close proximity to the chuck while the gas exiting from the chuck prevents contact between wafer and chuck. Small diameter chucks located close together help the present invention avoid distortion or fracture when holding very thin wafers for processing.
In addition, the chucks of the present invention cause the gas exiting therefrom to exit preferentially in a certain angular direction. Chucks are positioned in a non-contact wafer holder pursuant to the present invention so as to reduce or avoid high pressure zones due to build up of gas exiting from each chuck. Such local regions of higher pressure tend to distort wafers away from the plane of the holder, resulting in non-uniform etching or fracture of the wafer. In particular, the chucks of the present invention are arranged on the wafer holder such that exiting gas is preferentially directed radially towards the periphery of the holder. Furthermore, chucks are preferably arranged such that gas exiting from a typical chuck is directed between adjacent chucks, not directly at another nearby chuck. Chucks on the periphery of the holder are preferably positioned so as to have the gas exiting therefrom directed towards the periphery of the holder and overlapping the gas flow from immediately adjacent chucks. In addition, chucks on the periphery of the holder are preferably located as close together as feasible. The combination of overlapping gas flow and close proximity tends to create a gas shield on the boundary of the wafer holder, increasing edge protection for the wafer.
One embodiment of the present invention has all chucks supplied from a common gas supply, conveniently a reservoir to which all chucks are connected. Increased gas flow on the periphery of the wafer holder can be achieved by making use of larger diameter gas inlet orifices for such peripheral chucks. Smaller diameter gas inlet orifices for non-peripheral chucks tends to increase the vacuum attraction for such chucks. This differing gas flow for different chucks is desirable for providing protective gas flow on the periphery of the wafer holder and increased vacuum attraction near the center of the wafer holder. Large workpieces may be processed by means of the holder of the present invention, including rectangular, star-shaped or irregularly shaped workpieces.